innermost layer of digestive tract.
thin epithelial cell layer which is generally highly folded, creating more available surface area for absorption. The thin epithelium is supported by the submucosa - (both by muscle and connective tissue, which also supplies it with blood and contains the necessary lymph tissue) to assist the mucosa in warding off disease.

lumen

space within the GI tract tube

muscularis

Two or three layers of smooth muscle below the mucosa and submucosa, also contains nerve cells - is responsible for the localized contraction of the muscles, moves food along the GI tract.

DIGESTION

breaking food into smaller pieces so that it can be absorbed

absorption

final products of digestion cross the mucosal layer and enter the body via the bloodstream.

motility

controlled by hormones and nerves, this is the movement of food and food particles down the GI tract.

secretion

production of enzymes and hormones to assist in digestion and absorption. These are both exocrine (into lumen of GI tract, i.e. saliva, stomach acid, bile, pancreatic juice) and endocrine, i.e. hormones which regulate exocrine secretions. (Gastrin)

peristalsis

A pattern of muscle contractions which occur in sequential waves in order to move the contents of hollow organs around. Examples of peristalsis occuring in the body are in the esophagus to move food, in the ureters to move urine, and in the uterus to move a baby towards parturition. A co-ordinated muscular activity, it is controlled by nerves.

mouth

primary role is to break down food into smaller chunks. includes substructures of teeth and tongue.

produce pepsinogen, an inactive form of the enzyme pepsin, which degrades protein.

G cells of gastric glands

secrete a hormone, gastrin, directly into the bloodstream (ENDOCRINE), which aids in gastric juice secretion and gastric motility.

rugae

many ridges that the empty stomach folds into, but expand out when full.

intrinsic factor

B12 absorption substance secreted from parietal cells

pepsinogen

inactive form of pepsin

pepsin

protein degrading enzyme is released by gastric glands in inactive form. When in contact with acid activates. pepsin itself can make more pepsin when contacting pepsinogen.

gastric juice

watery fluid created by the combination of secretions of parietal and chief cells, contains hydrochloric acid, intrinsic factor and pepsinogen/pepsin.

gastrin

hormone that controls gastric motility & secretion of gastric juices.

secretin

intestinal hormone which inhibits gastric motility

cholecystokinin

Secreted from the duodenum, cholecystokinin (or CCK) is a hormone which is secreted into the bloodstream when proteins and fats are present in the duodenum. CCK works with secretin (also from the small intestine) to increase the rate of secretion of bicarbonates by the pancreas, and also works to contract the gall bladder while relaxing the sphincter of Oddi. These two actions together deliver bile into the small intestine. Cholecystokinin enables the small intestine to continue its work of breaking down of food into constituent parts.

STOMACH

large J shaped organ important in storage, mixing of foo, and digestion of fat and proteins. the stomach does NOT function in ABSORPTION. composed of three parts: fundus, body, and pyloris.

pancreas

functions to neutralize chyme and add digestive enzymes to the stomach.elongated lobular organ below pyloris of stomach, connected to the small intestine via the pancreatic duct.

watery solution containing bicarbonate and digestive enzymes secreted from the pancreas into the pancreatic and accessory exocrine ducts.

acinar cells

produce enzymes within the pancreas

bile

watery fluid contining bicarbonate, bile salts, cholesterol and bilirubin, synthesized by liver and stored in gallbladder. secretion control is by Cholecystokinin (CCK)

bile salts

synthesized in liver, cholesterol derivatives which facilitate digestion and absorption of fat, bicarbonate in bile neutralizes chyme to allow enzymes to work.

sphincter of Oddi

at the junction of the common bile duct and the small intestine, this sphincter prevents newly secreted bile from entering the small intestine if food is not present.

hepatic portal vein

takes venous blood to the liver from the entire GI tract.

gallstones

caused by the ability of the gallbaldder to concentrate bile through water removal (or if there is excessive bilirubin or cholesterol) this causes layers of cholesterol or bilirubin to be concentrated over time, forming large hard stones which can block the outflow of bile. (painful.)

bilirubin

secreted into the bile in a form which can not be absorbed by the intestine and is therefore disposed of. when the liver metabolizes hemoglobin molecules of destroyed red blood cells, bilirubin is one of the end products. excess bilirubin can cause jaundice: High levels of bile pigment are deposited in the mouth, nose and skin, causing yellowing discolouration. can occur because of health disorders such as sickle cell anemia, which causes too much bilirubin to be released to the blood, or impairment of the liver to remove bilirubin.

jejunum

most of the length of small intestine and therefore where most of the absorption occurs.

ileum

3rd part of the small intestine, empties into the large intestine at the ileocaecal sphincter. reabsorbs much of the bile salts, returned to the liver via the hepatic portal vein. absorbs B12.

villi

microscopic projections on surface of the folds of the small intestine, increases the surface area. layer of cells surrounding a central cavity with blood and lymph cells. gives intestine a HUGE absorptive surface.

small enough to be transported by "regular" cell transport methods, thes cross into the mucosal cells and then diffuse out into the blood.

carbohydrates

complex carbohydrates, tri and disaccharides, mostly broken down by salivary amylase, even in the stomach, then more in the intestine by pancreatic amylase. are broken into monosaccharides which can then cross into the mucosal cells.

monosaccharides

into mucous cells via secondary active transport. then diffuse out into blood.

micelles

small droplets of cholesterol and bile salts which are transported into the intestine from the liver & gallbladder. Free fatty acids enter micelles once broken down and are carried by them to the mucosal cells.

emulsify

break down large fat droplets into smaller fat droplets without changing the chemical composition of the fat.

chylomicron

carry monoglycerides and free fatty acids that have entered the mucosal cells via diffusion. Inside the ER of the mucosal cell, triglycerides are created from the monoglycerides and free fatty acids. these then have a protein coat them in the golgi apparatus, when they become a chylomicron. They are then packaged into secretory vessicles and are released into the lymph via the villus, as they are too large to go directly into the blood.

lipase

enzyme which reduces fat to free fatty acids and monoglycerides that can then enter the mucosal layer to be absorbed.

lymphatic system

system of lymph vessels that carry excess tissue fluid back to the circulatory system. Also aid in body defense.

water

small enough to enter the intestine and be absorbed without digestion, intestines can absorb ALL the water that you can drink. It diffuses easily, following the osmotic gradient.

cannot be absorbed unless attached to intrinsic factor. Absorbed in terminal ileum. If not enough intrinsic factor, not enough B12 and person can become anaemic as B12 required for RBC erythropoesis.

ileocecal valve

valve through which unabsorbed chyme enters large intestine (prevents backflow) also called ileocecal sphincter.

appendix

small outpouching of cecum - blind pouch at proximal end of large intestine. No digestive function but does contain lymph nodes.

haustra

series of sac-like pouches that join together to form the colon, the interior mucosal layer of each haustrum is smooth. Haustra function not in absorption, but as a food storage site. Food is moved around and mixed in the haustra by a process called haustral shuffling, and is acted upon, being broken down by bacteria present within the haustra.

bacteria

400+ species live in the large intestine. Digest substances not easily digestible by pancreatic fluids (i.e. fibre and fats.) produce gas in the large intestine and synthesize K vitamins.

K vitamins

synthesized by bacteria of large intestine, also absorbed from food by small intestine. important in production of clotting factora, and to metabolism. Fat soluble vitamin.

defecation

rectal distension leads to the defecation reflex which is the relaxation of the anal sphincters which allows the feces to be ejected out of the anus.

mass movements

sustained contractions of the colon in which one section contracts at a time, to drive colonic contents towards the rectum.

gastrocolic reflex

occurring after a meal, this is the increased motility of the colon which increases colonic movement towards the rectum.

external anal sphincter

can be voluntarily relaxed to allow defecation.

small intestine - structure facilitates function how?

The small intestine is segmented, allowing for contractions which allow the chyme to be moved along. Internally, it is highly folded, allowing for a greatly increased surface area, and therefore greatly increased absorption of materials. Each mucosal fold inside the small intestine is covered with finger-like villi, which in turn are covered with microvilli. Each villi has an internal area of blood vessels and lymph cells, into which the food products can be immediately absorbed, transported, and any harmful foreign particles destroyed. The chemical makeup of the material in the small intestine reflects its main purpose of breaking down food into the smallest possible parts, and tehn absorbing those parts into the bloodstream. The bile duct and pancreatic ducts empty into the small intestine, and other enzymes and lipases accompany the bile salts and residual acids to enable digestion and eventual absorption within this structure.

large intestine - structure facilitates function how?

The large intestine is more suited to slow, prolonged contractions (haustral shuttling, mentioned earlier) which mix its contents. It does not contain many folds, like the small intestine, but rather is smooth on the inside, with not much surface area. This, as well as its shorter length reflect its main purpose as a food storage area. Additionally, digestion aids are not emptied into the large intestine. Rather, it contains bacteria which enable further breakdown of the feces. It functions mainly to drive its contents towards the rectum for defecation, however does absorb any remaining ions and water.

breakdown of piece of bread from entry into mouth to absorption in small intestine.

Bile salts arrive as micelles or droplets in the small intestine and function to emulsify fats, breaking the size of fat droplets down into smaller droplets, without altering the molecular structure of those fats.

pancreatic lipases act to actually digest the emulsified (smaller) droplets, changing their chemical make-up by making them into fatty acids and monoglycerides that are then small enough to be absorbed by the intestine.

structural design of esophagus and stomach which ensure food only goes one way in GI system.

travel to the stomach where they are met with pepsin that has been secreted from the pancreas, which begins the protein breakdown. They become small enough that they can enter the mucosal cells, where they are further degraded by enzymes within those cells, to become small peptide chains. These are then transported through the mucosal cells by secondary active transport. They are further digested to amino acids by enzymes and can finally diffuse out to capillaries.

breakdown of carbohydrates.

digestion begins in the mouth by the enzymes there  salivary amylase. In the stomach, digestion continues via salivary amylase which is then joined by pancreatic amylase to continue carbohydrate breakdown from starches and glycogen (sugars) to di and tri-saccharides. These are then transported to the luminal cells in the intestinal membrane via secondary active transport, and after final break-down are transported as monosaccharides directly into the blood.

breakdown of fats

Stomach and intestinal peristalsis breaks fats into smaller droplets which are then further broken down (emulsified) by the bile salts from the liver and gall. Pancreatic lipases then break the fats down into monoglycerides and fatty acids. These can enter micelles, where they are transported by diffusion as they are soluble in the cell membrane. They are resynthesized there into triglycerides in the Endoplasmic reticulum, are coated with a protein layer by the golgi body, (now called chylomicrons) and enter lymph vessels where they are returned to blood, as theyre too large at this point to move directly into the blood through the vessels.